The present invention relates to an apparatus and method for a transition detector and pulse width qualifying circuit for a differential receiver. The incorporated U.S. patent application Ser. No. 7/480,426 provides for one embodiment of a transition detector and pulse width qualifying circuit. That patent application discloses a communications network comprised of Medium Attachment Units (MAUs) interconnected by a pair of twisted pair cables. The twisted pair cables transmit data packets and linkbeat pulses from one MAU to another according to the IEEE 802.3 10BASE-T standard, which is expressly incorporated by reference for all purposes. The standard requires that when one MAU is not transmitting a data packet to another MAU, it should periodically transmit linkpulses to the MAU to test the integrity of a link between the MAUs. FIG. 1 illustrates one example of one such packet having a preamble portion, a data portion and an end-of-transmission delimiter (ETD) portion, referred to as Manchester-encoded signal. The preamble portion is an alternating series of high and low transitions and the ETD is a high level for a specified minimum time duration.
FIGS. 2a and 2b illustrate two acceptable linkpulse transmit waveforms according to the IEEE Specification. A linkpulse waveform is either a high transition or a high transition followed by a low transition fitting within a specified pulse template.
The U.S. patent application Ser. No. 7/480,426 further discloses a smart squelch circuit which is a noise rejection apparatus to ensure that only the receipt of a Manchester encoded signal by a particular MAU will activate circuits of that MAU to operate upon the input signal in a predetermined manner. The squelch circuit requires the necessary information about the characteristics of the signal at the receive input to properly distinguish between Manchester signals, linkbeat pulses, and noise. The present invention provides an improved apparatus for determining and providing the required information about the input signal characteristics.